Collisions of Water with Helium: Calculations versus Experiment

Abstract

A detailed knowledge of the collisional excitation of water by He is very important for the modeling of water-rich regions in the interstellar medium. Since the pioneering calculations of the H2O-He potential energy surface (PES) and rotationally inelastic rate coefficients by Green et al [1], several improved calculations of PESs have been reported more recently [2-4]. Rate coefficients may be substantially different using one or another PES due to the high sensitivity of the dynamics to their features. We will report a combined experimental-theoretical study for the determination of the state-to-state rate coefficients of ortho- and para-H2O:He inelastic collisions in the 20-120 K thermal range. In the experiment, the H2O rotational populations along a supersonic jet of a mixture of He and highly diluted H2O are accurately measured using Raman spectroscopy. On the other hand, rate coefficients are computed by means of closecoupling calculations using the different PESs [2-4]. These calculations are tested against the experiments through a detailed analysis of the kinetic Master equation which combines both experimental and theoretical data. It is found that the rates based on the Hodges[2] and Patkowski[3] PESs agree with experiment within 15%, while the rates by Green[1] are between 25-40% too low. In this contribution, we will give an analysis of the different H2O-He PESs and their effects on the rotationally inelastic dynamics. This assessment will be completed by calculations of total integral cross sections in comparison with molecular beam experiments[5]. Finally, a preliminary estimation of water-water rate coefficients (affecting less diluted mixtures) will be reported.